Over the past several years, there has been considerable development in the use of fiber optic based systems for sensing applications. An important class of high sensitivity sensors has been proposed based upon interferometers. The basis of a fiber optic interferometer sensor is the measurement of a physical parameter through the modulation of the optical path length or phase that it induces, either directly or indirectly, in the optical fiber. External parameters that can directly modulate the optical path length of an optical fiber include temperature, strain and pressure. Indirect modulation of optical path length typically occurs via an auxiliary sensing element bonded to the fiber. For example, hydrophones have been proposed using a Mach-Zehnder interferometer in which light from a coherent source is divided into reference and sensing fiber optic cables. The sensing fiber optic cable is formed into a sensing coil that is exposed to an acoustic field that causes phase modulation of light passing through the sensing coil. The phase modulation occurs because the acoustic field causes changes in the diameter of the fiber core, the length of the fiber, and the refractive indices of the core and cladding, with the length change being the dominant effect. A second example of an interferometer sensor is a fiber optic gyroscope based upon the Sagnac effect.
There are two main classes of fiber optic magnetic field sensors, namely, those based on magnetostriction devices, and those based on Faraday rotation sensors. The distinction between the two sensor classes lies in the physical mechanism on which they are based. The Faraday effect is a linear process involving a direct interaction between the external magnetic field and the light propagating in the fiber. The magnetostriction effect exploited in fiber sensors, on the other hand, is a nonlinear interaction between the external field and the magnetostrictive material, resulting in a distortion or strain of the material. The strain is transferred to the fiber, where it can affect the optical path length of the fiber core.
As both the propagation constants and length of an optical fiber vary when it is subjected to either a changing temperature or force, it is possible to use a fiber optic cable as a sensor for measurements of strain, displacement and temperature. In a typical temperature sensor, a Michelson interferometer arrangement is used, with the sensing arm having a different length than the reference arm. Polarimetric configurations have also been proposed for fiber optic temperature sensors, based on the use of a length of birefringent fiber.
In general, fiber optic field sensors have achieved sensitivities exceeding that available from other technologies, as well as a very high dynamic range, due in part to the linearity of the phase change with the quantity being measured. However, despite high sensitivity and range, prior fiber optic sensors have been beset by a number of practical problems. First, the physical size of a fiber optic sensor can be significant, and can limit the applicability and versatility of the sensor. For example, it has been estimated that for a fiber optic hydrophone to be competitive with other hydrophone technologies, approximately one kilometer of monomode fiber would be required in the sensing arm to achieve the necessary sensitivity. Second, the components of a fiber optic interferometer are mechanically sensitive, and can introduce vibration noise that can deteriorate the signal-to-noise ratio. Another limitation of a fiber optic interferometer is the inconvenience in insuring the stability of the operating point of the interferometer, that is, in maintaining the quadrature condition between the two interferometer arms. This has been successfully implemented by using a PZT cylinder energized via a feedback loop, but this system has the inherent disadvantage that the length modulation range of the PZT is, at most, 2.pi.. There is thus a resetting transient when the phase differential to be corrected drifts outside this value. Temperature drifts are the prime cause of relative phase fluctuations, and only very small temperature differences can be handled without the onset of noticeable transients. A further major problem with all fiber optic interferometric sensors to date has been that the absolute relative phase difference between the sensing and reference paths is lost when the system is switched off. Until this data loss problem is solved, interferometric fiber optic sensors are likely to be used only in specialized applications.